The present invention relates to an actuator for an automatic performance piano.
In general, a well-known electromagnetic plunger is used as an actuator for driving keys of an automatic performance piano. The plunger normally includes an excitation coil, a movable iron core movably inserted through a bobbin of the excitation coil, a yoke having the excitation coil therein and forming a magnetic path, and the like. In accordance with an electrical signal reproduced from a recording medium, the plunger is driven to drive a key as if it were depressed by a human hand. A conventional actuator for this type will be described below. An actuator is disposed below the back ends of keys, and is constituted by a substantially U-shaped common yoke extending longitudinally along a direction in which the keys are arranged, and a plurality of electromagnetic plungers disposed inside the common yoke to correspond to the respective keys. The common yoke is made of a magnetic body such as soft iron so as to provide a common magnetic path for the respective electromagnetic plungers. The common yoke is almost horizontally disposed below the back ends of the keys to keep a predetermined interval between a keybed and the keys.
Each electromagnetic plunger includes a bobbin which integrally has a pair of upper and lower flange portions and around which an excitation coil is wound, a stationary yoke disposed at an upper portion inside the bobbin and fitted in a through hole provided at the upper surface of the common yoke, i.e., an upper yoke, and a shaft inserted into an inner hole of the stationary yoke through a guide bush and having a drive button mounted to its upper end and a movable yoke mounted to its lower end. The lower portion of the stationary yoke is formed to have an inverted frustoconical shape. A tapered hole is formed on the upper surface of the movable yoke so as to correspond to the inverted frustoconical shape of the stationary yoke, and a space between the tapered hole and the cone forms a magnetic gap. The movable yoke is slidably inserted into an insertion hole, provided to a lower yoke, and is attracted upward by a magnetic flux effect generated at the magnetic gap when the excitation coil is energized, so that the drive button together with the shaft are moved upwrd to push the back end lower surface of the key. Note that a stopper and buffer felts are also included.
In the actuator having the above structure, when a diameter of the bobbin is larger than a width of the key, portions of the bobbin extend from both sides of the key so that the adjacent bobbins interfere with each other. Therefore, bobbins are generally arranged to be alternately deviated from each other in the front-back direction, i.e., in a staggered manner. However, when the bobbins are arranged in such a staggered manner, a difference occurs in driving forces between front and back electromagnetic plungers, and a performance cannot be faithfully reproduced.
More specifically, when the electromagnetic plungers are horizontally arranged on the keybed to form front and back rows, an interval between the drive button of each electromagnetic plunger in the front row and the lower surface of the key is wider (by about 1.2 mm) than that between the drive button of each electromagnetic plunger in the back row and the lower surface of the key. Therefore, time lag occurs when the keys are driven, and the driving force of the electromagnetic plunger in the back row is larger than that of the electromagnetic plunger in the front row.
The conventional actuator has another problem wherein acceleration of the plunger is considerably large when it pushes the key upwardly. Thus, large impact noise is generated when a key is stricken. Furthermore, in the conventional actuator, it is troublesome and takes a long time to mount the electromagnetic plunger to the mounting plate, resulting in poor assembly workability.